The invention broadly relates to a process for preparing high mechanical strength pure silicon dioxide from hexafluorosilicic acid and ammonia.
An already known process for preparing silicon dioxide starting from hexafluorosilicic acid and ammonia is described in the U.S. Pat. No. 3,271,107. In that process a 15-25% by weight hexafluorosilicic acid solution is reacted with an ammonium hydroxide solution, having a concentration of, for instance, 29% by weight, in a first step in a reaction zone with firm stirring at a pH of 6.0-8.0 to produce a suspension or slurry containing numerous extremely small silica nuclei. The quantities of the two reactants must be controlled to insure that the slurry has a pH of 6.0 to 8.0. The suspension or slurry formed in this reaction contains silicon dioxide and is subsequently reacted with ammonium hydroxide in a second step with less vigorous stirring for 20-120 minutes, at a pH of 8.3-9.0 to form a precipitate. The precipitated silicon dioxide is then separated off. The thus obtained silicon dioxide is a flocculate that in practice may only be used in low-grade technical applications, such as, for example, a filler in rubber products or as an additive for pigments. It has now been found impossible from a practical point of view to convert the thus obtained silicon dioxide into small lumps of sufficient mechanical strength and of reasonable dimensions which can be used for high-grade technical purposes, for instance as supporting material for catalysts.
U.S. Pat. No. 4,026,997 describes a process for preparing ammonium fluoride. In such process a silicon dioxide by-product is obtained by converting a 25-35% by weight solution of hexafluorosilicic acid with a 15-20% by weight ammonium hydroxide solution at 40.degree. C. to 90.degree. C. until a suspension is obtained containing more than 1% by weight free ammonia, less than 22% by weight ammonium fluoride, and precipitated silicon dioxide. The silicon dioxide by-product consists of coarser particles than in the above-disclosed U.S. patent, but the particles are extremely weak. Indeed, it has proved impossible to convert them to particles of sufficient mechanical strength.
A further frequently applied process for preparing silicon dioxide in the form of strong particles having reasonable particle size dimensions suitable for technical purposes consists of preparing silicon dioxide in the form of a gel (silica gel) by acidifying sodium silicate. The sodium salt is then washed out of the gelatinous precipitate obtained and a silica hydrogel is formed. This hydrogel can be converted by drying to form a xerogel. This xerogel is known in the trade as silica. In general this silica may be used, for instance, as supporting material for catalysts and as a drying agent, because it is obtained in the form of small strong lumps with reasonable dimensions.
Disadvantageously, however, the silica-gel process is very expensive. The complete washing of the sodium salt is a very expensive but necessary process if the silica is to have any real technical utility since sodium-containing silicon dioxide is known to sinter readily even at low temperature. Sintered silica loses its desirable surface qualities and characteristics.
Disadvantageously other preparative processes starting from hexafluorosilicic acid yield silicon dioxide in the form of a flocculate. The flocculate cannot be converted into a gel (see, for instance: `The Chemistry of Silica` by Iler, R. K., page 563, John Wiley & Sons Inc., New York, 1979. The flocculate obtained in the processes applied so far for the preparation of silicon dioxide from hexafluorosilicic acid and ammonia consists of small, very weak particles, which cannot be converted into particles having sufficient strength and reasonable dimensions.
The present invention now provides a process using a hexafluorosilicic acid solution starting material to produce silicon dioxide having a low sodium content in the form of small lumps of reasonable dimensions and great mechanical strength. This silicon dioxide produced in accordance with the present process has a low sodium content and combines a high attrition resistance and mechanical strength with a relatively large surface area and pore volume. This silicon dioxide product has superior qualities imparted to it by the present process which qualities exceed those associated with the generally used alpha-alumina catalyst supports. Moreover, it has now been found that the thus obtained hydrophobic product can be economically and easily converted into a hydrophilic product which has about the same desirable qualities and characteristics.